This invention relates to an opto-electronic scale reading apparatus. It may be used in an encoder for measuring linear or angular displacement of one member relative to another.
A known type of such scale reading apparatus is described in European Patent Application No. 207121. Periodic marks on a scale are illuminated and act as a periodic pattern of light sources. A readhead comprises, in succession from the scale, an index grating, an analyser grating and a sensor assembly. Alternatively, the analyser grating and the sensor can be integrated, as described in European Patent Application No. 543513, giving what may be termed an xe2x80x9celectrogratingxe2x80x9d.
The devices described in those patent applications rely on diffraction which takes place in the readhead, not at the scale. In particular, use is made of the diffraction phenomenon known as xe2x80x9cself-imagingxe2x80x9d or xe2x80x9cFourier imagingxe2x80x9d of periodic transmission masks. Fringes are formed at the level of the analyser grating, by Fourier imaging of the index grating. When he scale and readhead move relative to each other, in the longitudinal direction of the scale, these fringes also move. Detection of the fringe movement provides a measure of the relative displacement of the scale and the readhead.
In practice, such devices are sensitive to the spacing or xe2x80x9cride heightxe2x80x9d between the scale and the readhead. When the scale and readhead are installed on a machine, it is necessary to ensure that the readhead will remain at an appropriate ride height throughout the length of its travel along the scale.
Our International Patent Application No. WO96/18868 addresses this problem, and decreases the ride height sensitivity, but at the expense of an overall decrease in the contrast (and therefore visibility) in the resulting fringes.
The present invention is based upon new research by the inventor into the mechanism of operation of readheads of the type described in EP 207121, and consequent insights into the causes of the ride height sensitivity.
The fringe visibility is found to depend both upon a Talbot fringe visibility envelope, and upon a geometric fringe visibility envelope which depends upon the size of the light source.
According to one aspect of the invention, the light source is restricted to a small but finite size, and positioned so that it subtends a small angle at the analyser grating (or, if a collimating lens is used, positioned so that it subtends a small angle at the lens). The size of the light source should preferably be smaller than a predetermined value such that the extent of the geometric fringe visibility envelope exceeds the extent of the Talbot fringe visibility envelope.
According to a further aspect of the present invention, the scale is illuminated with collimated light. This may be done by placing a collimating lens between the illuminating source and the scale. As a result of the collimated illumination, the fringes have a constant pitch rather than diverging.